Mental crash

I moved back to uni this weekend and into a house with my two housemates - I lived with them both in halls last year, so I know what they're like to live with. I love them both, but since arriving back on friday, im feeling awful. I feel really low, have no energy, everything is making me anxious and I feel like crying all the time. I have no idea why though, I love uni. 
I'm not yet officially diagnosed, but ive been seeing a therapist, who says that im definitely on the spectrum and that a diagnosis is just a formality at this point. I have the appointment for diagnosis tomorrow. I'm already feeling highly emotional right now and like I can't cope. I feel like I need the official diagnosis and I've been waiting for ages - the uncertainty is killing me - but im worried that tomorrow i'll be really stressed out and emotional, since im going to spend the whole afternoon talking about the things that stress me out the most. 
Normally, this wouldn't be too much of a problem for me, I'd just shut myself in my room and cry in bed, but my housemates are having a house party for the next three days, for one of their birthdays. I nornally hate parties anyway, but the thought of people in my space whilst I feel like I do at the moment is unbearable. I'm worried I'm going to do or say something that i'll regret. Please help!